In traditional cellular systems, each cell transmits to its own user equipment (UE) and in the process creates interference to UEs in adjacent cells. This is illustrated in FIG. 1A. However, in coordinated multipoint transmission/reception (CoMP) based systems, multiple base stations (BSs) can coordinate to transmit to a UE as shown in FIG. 1B. There is rate gain from both coordination and also elimination of interference. “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Further Advancements for E-UTRA Physical Layer Aspects (Release 9), 3GPP TR 36.8.14 V0.4.1 (2009-03)” teaches that CoMP strategies fall into the following two strategies.
a) Coordinated Scheduling and/or Beamforming (CS/CB)    i) Data to a single UE is transmitted from one of the transmission points where a transmission point means a BS in the set of cooperating BSs.    ii) Scheduling decisions at different transmission points are coordinated to control the interference generated.
b) Joint Processing/Transmission (JP)    i) Data to a single UE is simultaneously transmitted (coherently or non-coherently) from multiple transmission points to improve the received signal quality and/or cancel actively interference for other UEs.
The first step in any CoMP scheme, such as CS/CB or JP, is to decide which BS will cooperate for a given UE. In “R1-092232, Summary of email discussions CoMP v2”, 3GPP TSG RAN WG1 meeting #57″, the following terms are introduced in this context.    i) CoMP measurement set: Set of cells for a given UE whose link gains matrices are measured by the UE.    ii) CoMP Cooperating set: Set of cells that cooperate to transmit to a given UE.In CS/CB user data need not be shared amongst cells in the cooperating set but in JP data needs to be shared.
The layout of cells is shown in FIG. 2. This is a model which is widely used for analysis and simulation of cellular systems as it is a close approximation of the real physical layout. It is considered that a 57 cell network organized in groups of 19 such that the base stations of the 3 cells within each group are co-located. Each of the 3 co-located base stations use a directional antenna that confines the majority of its transmitted signal energy into a sector of 120 degrees and the three sectors from three cells are non-overlapping. When there is no coordination, each UE is served by its serving cell. In case of CoMP, groups of cells can serve a given UE. Such groups can be formed either in a static way depending on cell geometry or dynamically, based on the link gains of the UEs to the cells.
In a static cooperating set formation, the clusters of cells are formed apriori to the UE signaling and reporting of link gain values to different cells. The formation of clusters is based on geometry. FIG. 3 shows two such configurations. In FIG. 3A, adjacent sectors belonging to different cell sites coordinate while in FIG. 3B, the three sectors belonging to the same cell site coordinate. For graphical clarity only one coordinating set is illustrated in the figure, the others can be obtained by repeating the pattern over the entire cellular area. In each case the cooperating set consists of three sectors and they serve all the UEs that are associated with any one of the three sectors. Static cooperating sets have been considered by many references, such as R1-0933177, “DL performance evaluation for ITU-R submission”, 3GPP TSG RAN WG1 meeting #5″, R1-093585, “A Simple Coordinated Scheduling Algorithm for Downlink CoMP”, 3GPP TSG RAN WG1 meeting #58″, and “H. Huang and M. Trivellato, “Performance of multiuser MIMO and network coordination in downlink cellular networks””.
The problem with static cooperating set formation is that, for a given UE, the cells that have the strongest link gain to it after its serving cell (i.e. the cell that serves it in case of no CoMP), may not lie in its cooperating set. These cells would then cause strong interference. This is possible due to the random nature of link gain of which the random shadowing component is the biggest factor. Thus, in another way of forming the cooperating set of a given UE, the UE first determines the cells with the strongest link gains (by measuring the Reference Signal Received Power or RSRP of all cells and finding out which ones lie within a threshold of the serving cell) and then reports this to its serving cell. The serving cell then decides to form a cooperating set with these cells. Such approaches have been considered in “R1-093410, “Coordinated Beamforming Algorithms Based on Spatial Covariance Feedback and its Gain over Single-point SU/MU Beamforming”, 3GPP TSG RAN WG1 meeting #58”.
Frequency partitioning is another important method that increases the overall system performance. One simple illustrative example could be that the total available spectrum is partitioned into disjoint bands to transmit to different classes of UEs. This mitigates the problem of interference and allows each cell to perform SU MIMO algorithms in these disjoint bands. The other alternative would have to transmit to the different UEs in the total band but use MU-MIMO beamforming to mitigate interference. The former approach of frequency partitioning can be easier to implement. Often different frequency bands are used for transmitting to uses in the cell edge as they are the most prone to interference. Such approaches have been considered in “R1-093279, “Downlink CoMP based on cooperative precoding”, 3GPP TSG RAN WG1 meeting #58”.
Coordinated Multipoint Transmission/Reception (CoMP) is considered to be an important feature for Release-10 (Rel-10) LTE-Advanced technology as this enhances the system data rate and reliability. CoMP involves multiple cells cooperating together to transmit to a single UE. In the future it is envisaged that there will be networks which will have a mixture of Release 8 (Rel-8) LTE UEs that do not support CoMP and Rel-10 UEs that have the option of using the CoMP mode, especially during the period of transition when the market moves from the current LTE stage to the future LTE-A stage.
If a UE is using the CoMP mode, the transmission set or the cells that cooperate to transmit to it, needs to be decided. Instead of forming fixed transmission sets based on cell geometry, it is better to form these dynamically based on the UE link gains to the various cells (R1-093410, “Coordinated Beamforming Algorithms Based on Spatial Covariance Feedback and its Gain over Single-point SU/MU Beamforming”, 3GPP TSG RAN WG1 meeting #58).
At each Transmission Time Interval (TTI), the BS of a cell has to decide to which UE to transmit to amongst all UEs for which it is the serving cell. One approach would be to transmit to that UE to which it can support the highest data rate, which is the UE which has the best link gain to the BS. However as link gains are strongly dependant on distance, this means that most of the times the nearby UEs are scheduled. Thus this scheduling method is said to be unfair to the faraway UEs. To remedy this situation, the proportional fair (PF) metric has been introduced and is widely used in many wireless systems such as CDMA/HDR. In this method the BS calculates the instantaneous rate to a UE if that UE is scheduled and divides it by the average rate received by the UE till that TTI. The UE with the highest value of this metric is chosen for transmission. Thus in PF scheduling both instantaneous rate (strength of link gain) and average rate (long term fairness) is considered.
Thus, there is a need for efficient portioning of BS resources, such as time, bandwidth and also assigning BSs to cooperating sets of different UEs so that there is seamless operation of both Rel-8 and Rel-10 UEs.